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Re: bNodes wanted

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Fri, 24 May 2002 12:36:28 -0400
To: sandro@w3.org
Cc: Andy_Seaborne@hplb.hpl.hp.com, me@aaronsw.com, www-rdf-interest@w3.org
Message-Id: <20020524123628M.pfps@research.bell-labs.com>
From: Sandro Hawke <sandro@w3.org>
Subject: Re: bNodes wanted 
Date: Fri, 24 May 2002 11:48:33 -0400

[...]

> The only time you change the meaning of
> RDF via Skolemization is when the RDF is not asserted.   As I think
> you pointed out it is nice to know no-one else has the node, but
> that's just an engineering convenince.

>     -- sandro

The above is not true.


Skolemization does change the meaning of RDF.   For example, an RDF
interpretation can satisfy 

_:0 <http://a.b/x> "hi" .

without satisfying

<http://a.b/new> <http://a.b/x> "hi" .

This difference in meaning has entailment consequences, as the first triple
is entailed by the second, but does not entail the second.


What then is true then about Skolemization?

Well, Skolemization usually doesn't change whether a logical construct is
satisfiable.  But this isn't very interesting in RDF as every RDF graph is
satisfied by some RDF interpretation, at least in the current model theory.

Also, if u is a URIREF that does not occur in RDF graphs P and C, then P
RDF-entails C exactly and only when P-with-some-bnode-in-it-skolemized-as-u
RDF-entails C.  However, remember that there is no way that you, as an
author of some part of P, can ensure that any particular URIREF will never
occur elsewhere, so every time you Skolemize a bnode, you are changing
its meaning in ways that can be observed from the outside.

Peter F. Patel-Schneider
Received on Friday, 24 May 2002 12:38:08 GMT

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